1. Field of the Invention
The invention relates to a signal processing apparatus and so on for a radar transceiver, which receives a reflected signal generated by a target object in response to a frequency modulated transmission signal and generates a beat signal having a frequency difference between the transmission signal and a reception signal, and more particularly to a signal processing apparatus and so on that detects a target object on the basis of a peak signal in a frequency spectrum of the beat signal.
2. Description of the Related Art
In recent years, Frequency Modulated-Continuous Wave (FM-CW) type radar apparatuses have been installed for use in vehicles as obstruction detecting means used during vehicle collision avoidance/collision response control. Japanese Patent Application Publication No. 11-271433 (JP-A-11-271433) describes an example of a vehicle-installed FM-CW type radar apparatus.
A vehicle-installed FM-CW type radar apparatus implements frequency modulation on a millimeter wavelength continuous wave (electromagnetic wave) in accordance with a triangular wave-shaped frequency modulation signal, transmits the result to a search area, and receives a reflected signal generated by a target object. At this time, a frequency of the reflected signal shifts due to the effects of a time delay corresponding to a relative distance of the target object and a Doppler shift corresponding to a relative velocity of the target object, and therefore a frequency difference occurs between the transmission and reception signals. To detect the frequency difference, the radar apparatus mixes the transmission and reception signals, thereby generating a beat signal having a frequency (beat frequency) that corresponds to the frequency difference between the signals.
When a plurality of target objects having different relative distances or different relative velocities exist in the search area, a reflected signal having a different frequency is included in the reception signal for each target object. Therefore, in a frequency spectrum obtained by subjecting the beat signal to Fast Fourier Transform (FFT) processing, a maximum value is formed at a different frequency for each target object. Hereafter, a beat signal forming a maximum value will be referred to as a peak signal, and a beat frequency thereof will be referred to as a peak frequency.
The radar apparatus detects an azimuth angle, the relative distance, and the relative velocity of each target object using a phase and the peak frequency of the peak signal, and outputs a detection result to a vehicle control apparatus that controls the behavior of the vehicle. The vehicle control apparatus then determines a collision probability on the basis of the relative velocity, relative distance, or azimuth angle of the plurality of detected target objects, and when a collision is anticipated, the vehicle control apparatus drives various actuators for performing a collision avoidance operation or a passenger protection operation.
Here, the number of target objects for which the collision probability can be determined by the vehicle control apparatus within a limited amount of time is limited by the throughput of the vehicle control apparatus. Therefore, when a plurality of peak signals are detected, the radar apparatus outputs a detection result relating to an important target object in terms of vehicle control to the vehicle control apparatus preferentially, rather than outputting the detection results relating to all of the peak signals. For this purpose, the radar apparatus extracts a peak signal representing this target object preferentially after detecting a number of peak signals corresponding to the number of target objects from the beat signal, and detects the target object on the basis of the extracted peak signal.
In the case of a radar apparatus that monitors the front of the vehicle, a target object having a high collision probability, or in other words a target object positioned at a short distance from a front surface of the vehicle, is considered to be an important target object. Various methods of extracting the peak signal that represents this target object have been proposed.
A first method focuses on the fact that, according to an antenna pattern of the radar apparatus, a reflected signal from the front surface of the vehicle has a maximum level, and therefore a peak signal having a large level is extracted preferentially. A second method focuses on the fact that the peak frequency decreases as the relative distance of the target object decreases, and therefore a peak signal having a low peak frequency is extracted preferentially.
However, the methods described above exhibit the following problems.
FIG. 1 is a view illustrating positional relationships between a radar apparatus and target objects, and the condition of peak signals respectively representing the target objects. For ease of description, a case in which one peak signal is extracted preferentially from peak signals representing two target objects is used here as an example.
First, as shown in FIG. 1A, when a target object T1 (a passenger vehicle, for example) is positioned at a short distance from the front surface of the radar apparatus and a target object T2 (a large truck, for example) having a larger reflection sectional area is positioned in front of the target object T1, peak signals representing the target objects T1, T2 are as shown in FIG. 1B. Here, considering the importance in terms of vehicle control, the target object T1 should be extracted preferentially, but when the method of extracting the peak signal having the largest level preferentially is employed, a level L2 of a peak signal P2 representing the target object T2 having the larger reflection sectional area exceeds a level L1 of a peak signal P1 representing the target object T1, and therefore the peak signal P2 representing the target object T2 is extracted preferentially.
According to the method of extracting the peak signal having the lowest peak frequency preferentially, a relative distance RI of the target object T1 is smaller than a relative distance R2 of the target object T2, and therefore a peak frequency α1 of the peak signal P1 is lower than a frequency α2 of the peak signal P2. Hence, the peak signal P1 is extracted preferentially. As shown in FIG. 1C, however, when the target object T2 is positioned extremely close, for example in the adjacent lane, a relative distance R21 of the target object T2 becomes smaller than the relative distance R1 of the target object T1. In this case, as shown in FIG. 1D, a frequency α21 of the peak signal P2 is lower than the frequency α1 of the peak signal P1, and therefore the peak signal P2 is extracted preferentially.
Hence, with the methods described above, the peak signal representing the target object that has a high level of importance in terms of vehicle control and is positioned in front of the vehicle at a short distance from the front surface of the vehicle may not be extracted, and as a result, the corresponding target object may not be detected.